Axle assembly with electrically-driven differential

ABSTRACT

An axle assembly for a motor vehicle. The assembly comprises a carrier housing and a differentially assembly rotatably mounted to the carrier housing. A ring gear is fixed to the differential assembly. A first pinion is in meshing engagement with the ring gear. The first pinion is driven by a combustion engine and is operable to drive the ring gear. An electric drive assembly is mounted to the carrier housing. The electric drive assembly comprises an electric motor and a second pinion. The second pinion is in meshing engagement with the ring gear. The second pinion is driven by the electric motor and is operable to drive the ring gear.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 10/978,870filed Oct. 29, 2004, now U.S. Pat. No. 7,115,058, the entire disclosureof which is incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to automotive axle assemblies. Inparticular, the present invention relates to a differential carrierassembly having an electric drive assembly.

BACKGROUND OF THE INVENTION

Automobile manufacturers are increasingly offering for sale automobilespowered by electric motors. Electric motors provide numerous advantageswhen compared to traditional combustion engines. For example, electricmotors typically operate more efficiently and emit fewer undesirableemissions to the environment. However, combustion engines generallyoutperform electric motors in numerous other performance categories andtypically provide higher horsepower per pound when the weight ofbatteries are taken into consideration. Further, automobiles propelledby electric motors are generally more expensive than automobilespropelled by combustion engines and need to be recharged more often thana vehicle propelled by a combustion engine needs to be refueled.Therefore, in spite of the advantages associated with electric motors,consumers are generally reluctant to purchase motor vehicles propelledsolely by electric motors.

Accordingly, there is a need in the art for a motor vehicle that offersthe advantages of an electric engine, such as increased efficiency, aswell as the advantages of a combustion engine, such as reduced cost andenhanced performance.

SUMMARY OF THE INVENTION

In one form, the present invention provides for an axle assembly for amotor vehicle. The assembly comprises a carrier housing and adifferential assembly that is rotatably mounted to the carrier housing.A ring gear is fixed to the differential assembly for rotationtherewith. A first pinion is in meshing engagement with the ring gear.The first pinion is adapted to be driven by a combustion engine and isoperable to drive the ring gear. An electric drive assembly is mountedto the carrier housing. The electric drive assembly includes an electricmotor and a second pinion. The second pinion is in meshing engagementwith the ring gear. The second pinion is selectively driven by theelectric motor and is operable to drive the ring gear.

In another form, the present invention provides for an axle assembly fora motor vehicle. The axle assembly comprises a carrier housing and aring gear rotatably mounted within the carrier housing. A first pinionis in meshing engagement with the ring gear and is operable to drive thering gear. A second pinion is in meshing engagement with the ring gear.The second pinion is also operable to drive the ring gear. An electricmotor is mounted to the carrier housing. A controller monitors theoperating parameters of the motor vehicle. The controller selectivelyactivates and deactivates the electric motor in response to the statusof the operating parameters.

In yet another form, the present invention provides for a method forpropelling a motor vehicle having both an internal combustion engine andan electric motor. The internal combustion engine is selectively coupledto a ring gear. The ring gear is rotatably mounted within a carrierhousing. The electric motor is selectively operable to provide drivetorque to the ring gear. The method comprises the following steps:drivingly interconnecting the ring gear and the combustion engine;drivingly interconnecting the ring and the electric motor; monitoringpredetermined operating parameters of the motor vehicle using acontroller; and selectively activating and deactivating the electricmotor based on the status of the predetermined operating parameters.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of a motor vehicle constructed inaccordance with the teachings of the present invention;

FIG. 2 is a fragmentary perspective view of a portion of the motorvehicle of FIG. 1, illustrating the rear axle in greater detail;

FIG. 3 is a section view of a carrier assembly of the motor vehicle ofFIG. 1 equipped with an electric drive assembly according to anembodiment of the present invention;

FIG. 4 is a section view of a carrier assembly of the motor vehicle ofFIG. 1 equipped with an electric drive assembly according to anotherembodiment of the present invention; and

FIG. 5 is a section view of a carrier assembly of the motor vehicle ofFIG. 1 equipped with an electric drive assembly according to anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiments is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

With reference to FIG. 1, a vehicle having an axle assembly constructedin accordance with the teachings of the present invention is generallyindicated by reference numeral 10. The vehicle 10 includes a driveline12 drivable via a connection to a power train 14. The power train 14includes a combustion engine 16 and a transmission 18. The driveline 12includes a drive shaft 20, a rear axle 22, and a pair of left and rightwheels 24 a and 24 b, respectively. The rear axle 22 includes anelectric drive assembly 26. A power supply, such as an alternator 28 ora battery 29, supplies electricity to the electric drive assembly 26. Acontroller 30 monitors inputs from sensors 32 to determine variousoperating parameters of the vehicle 10, such as vehicle speed, enginerevolutions per minute (RPM), ring gear speed, throttle position asindicated by the position of an accelerator pedal 31, and otherparameters as set forth below. The controller 30 processes theinformation gained using the sensors 32 and, as described further below,operates the electric drive assembly 26 based on this information.

FIGS. 2 and 3 depict the rear axle 22 in greater detail. Rear axle 22includes a carrier assembly 34, a left axle shaft assembly 36, and aright axle shaft assembly 38. The carrier assembly 34 includes a carrierhousing 40, a differential unit 42, and an input shaft assembly 44. Thehousing 40 supports the differential unit 42 for rotation about a firstaxis 46 and further supports the input shaft assembly 44 for rotationabout a second axis 48 that is perpendicular to the first axis 46.

The housing 40 is initially formed in a suitable casting process andthereafter machined as required. The housing 40 includes a main body 50and a pinion nose 52 extending from the main body 50. The housing 40includes a wall member 54 that defines a central cavity 56 having a leftaxle aperture 58, a right axle aperture 60, and a first pinion aperture62.

The left axle shaft assembly 36 includes a first axle tube 70 fixed tothe left axle aperture 58 and a first axle half-shaft 72 that issupported for rotation about the first axis 46 by the first axle tube70. Similarly, right axle shaft assembly 38 includes a second axle tube74 that is fixed to right axle aperture 60 and which supports a secondaxle half-shaft 76 for rotation about the first axis 46.

FIG. 3 depicts the differential unit 42 being disposed within thecentral cavity 56 of the main body 50 of the housing 40. Thedifferential unit 42 includes a case 80, a ring gear 82 that is fixedfor rotation with the case 80, and a gear set 84 that is disposed withinthe case 80. Ring gear 82 includes a plurality of teeth 83. The gear set84 includes first and second side gears 86 and 88 and a pair ofdifferential pinions 90. Differential pinions 90 are rotatably supportedon pinion shafts 92 that are mounted to the case 80. The case 80includes a pair of trunnions 94 and 96 and a gear cavity 98. A pair ofbearing assemblies 100 and 102 support the trunnions 94 and 96,respectively, for rotation about the first axis 46.

First axle half shaft 72 and second axle half shaft 76 extend throughleft and right axle apertures 58 and 60 respectively, where they arecoupled for rotation about first axis 46 with first and second sidegears 86 and 88, respectively. The case 80 is operable to supportdifferential pinions 90 for rotation within the gear cavity 98 about oneor more axes that are perpendicular to the first axis 46. The first andsecond side gears 86 and 88 each include a plurality of teeth 104 thatmeshingly engage teeth 106 that are formed on differential pinions 90.

The input shaft assembly 44 generally includes a first input pinion 110,a propeller shaft coupling flange assembly 112, and a pair ofconventional bearing assemblies 114 and 116. Each of the bearingassemblies 114 and 116 include an outer race that engages the housing 40in a press-fit manner. The bearing assemblies 114 and 116 cooperate withthe housing 40 to support the first pinion 110 for rotation about thesecond axis 48.

The first pinion 110 includes a tail portion 118 and a head portion 120integrally formed with one another. The head portion 120 includes aplurality of teeth 122. The teeth 122 meshingly engage the teeth 83 ofthe ring gear 82 to transfer torque from the power train 14 to thedifferential unit 42. A plurality of external splines 119 are formed onthe tail portion 118. An externally threaded portion 121 is formed on areduced diameter section at the end of the tail portion 118.

The coupling flange assembly 112 generally includes a pinion flange 124and a seal assembly 126. The pinion flange 124 includes an internalspline 128 in engagement with external splines 119 of the first pinion110. A nut 130 fixedly mounts the pinion flange 124 to the threadedportion 121 of the tail portion 118. At least a portion of the pinionflange 124 is located within the pinion nose 52.

The seal 126 is mounted at the first pinion aperture 62 by any suitablemounting means. The seal 126 includes an aperture 132 through which thefirst pinion 110 passes. The seal 126 is any suitable seal operable toprevent the passage of foreign materials, such as dirt and fluids, fromentering the housing 40.

The electric drive assembly 26 includes a support casing 150. The casing150 can be integral with the carrier housing 40 or secured to thecarrier housing 40 using any suitable fastening means. For example, andas illustrated in FIGS. 3 through 5, the casing 150 can be secured tothe housing 40 by bolts 152 that extend through an outer wall 154 of thecasing and engage the wall member 54 of the housing 40. The casing 150can be secured to the housing 40 at a variety of different locations, aslong as cooperation between components of the electric drive assembly 26and the ring gear 82 is possible. The casing 150 can be made from anynumber of suitable materials, such as aluminum, steel or compositematerial.

The housing 40 includes an opening 156 to allow components of theelectric drive assembly 26 to cooperate with the ring gear 82. Theopening 156 can be specifically formed to accommodate the position ofthe assembly 26 or the opening 156 can be a pre-existing opening, suchas a conventional carrier cover pan opening as illustrated. When theopening 156 is the conventional cover pan opening, the casing 150 can besecured to the housing 40 using the same bolt holes used to secure aconventional cover pan (not shown) to the cover pan opening.

The electric drive assembly 26 depicted in FIG. 3 generally comprises asecond pinion 160, an electric motor 162, and a clutch assembly 164. Thesecond pinion 160 generally includes a second pinion tail portion 170and a second pinion head portion 172. The tail 170 of the second pinion160 is integrally formed with the head portion 172. The head portion 172includes a plurality of gear teeth 174. The tail 170 is rotatablymounted in the support casing 150 on one or more bearings 176 to permitrotation of the second pinion 160 about a third axis 175 that can bealigned with the second axis 48, as illustrated in FIG. 3. The secondpinion 160 is mounted in a position such that the teeth 174 of thesecond pinion 160 are in meshing engagement with the teeth 83 of thering gear 82. The gear teeth 174 can be of any suitable shape or sizethat allows the teeth to meshingly engage the teeth 83 of the ring gear82, which also can be of any suitable shape or size. Furthermore, asillustrated, the axis 175 may be offset from axis 46 to form a hypoidgear arrangement.

The motor 162 and the clutch 164 are both seated within a motor housing178. The motor housing 178 is rigidly coupled to an interior of thesupport casing 150 in any suitable manner. The motor housing 178 canalso be integral with the casing 150. The motor housing 178 includes aninterior aperture 180 that extends through the approximate center of themotor housing 178. The tail 170 extends through the interior aperture180. The motor housing 178 also includes cooling fins 182. The coolingfins 182 are located on an outer surface of the housing 178 to increasethe surface area of the motor housing 178 and assist in cooling thecomponents of the housing 178, such as the electric motor 162 and theclutch 164 assembly.

The motor 162 is integral with or mounted within the motor housing 178using any suitable mounting means. The motor 162 can be any suitableelectric motor. The motor 162 includes a rotor 184. The rotor 184 isgenerally cylindrical and has a center aperture. The rotor 184 isrotationally mounted within the housing 178. The rotor 184 rotates aboutthe third axis 175. The pinion tail 170 extends through the centeraperture of the rotor 184. Thus, the rotor 184 may rotate about thepinion tail 170. The motor 162 is powered by any suitable power source,such as the alternator 28 or the battery 29.

The clutch assembly 164 generally includes a clutch pack 190 and anactuator 192. The clutch pack 190 includes a series of first clutchplates 194 and a series of second clutch plates 196. The first clutchplates 194 are splined to the tail 170 of the second pinion 160 and thesecond clutch plates 196 are splined to the rotor 184. The first andsecond clutch plates 194 and 196 are interleaved with one another. Thefirst and second clutch plates 194 and 196 are axially slidable alongthe third axis 175. The actuator 192 is operable to axially slide thesecond clutch plates 196 into engagement with the first clutch plates194. The actuator 192 can be any conventional actuator. For example, theactuator 192 can be a hydraulic actuator or an electric actuator, whichcould be powered by the alternator 28 or the battery 29. When the firstand second clutch plates 194 and 196 engage each other, rotation of therotor 184 effectuates rotation of the second pinion 160.

The controller 30 monitors various operating parameters of the vehicle10 and controls the operation of the electric drive assembly 26. Thecontroller 30 can by any suitable signal processing device or controldevice, such as a microcomputer. The controller receives inputs fromvarious sensors 32, which monitor different parameters of the vehicle10. The sensors 32 can each be any suitable conventional sensing device.Specifically, and as shown in FIGS. 1 and 3, the sensor 32A monitors thespeed of the engine 16. The sensor 32B monitors the revolutions of thedrive shaft 20 to determine the ground speed of the vehicle 10. Thesensor 32C monitors the rotational speed of the ring gear 82. The sensor32D monitors the rotational speed of the second pinion 160. The sensor32E monitors the operation of the electric motor 162, the position ofthe clutch 164, and the position of the actuator 192. The sensor 32Fmonitors the throttle position as an indication of the amount ofacceleration desired by the operator. The controller 30 is also operableto send commands to the engine 16 and the electric drive assembly 26 tocontrol their operation.

The controller 30 activates the electric drive assembly 26 when certainpredetermined operating conditions of the motor vehicle 10 are present.In some applications, the controller 30 is programmed to activate theelectric drive assembly 26 when the vehicle 10 is accelerating from astandstill, such as between 0 mph and 10 mph, and at highway cruisingspeeds, such as above 55 mph. However, the controller 30 can beprogrammed to activate the electric drive assembly 26 in response to theoccurrence of a variety of different conditions.

Operation of the electric drive assembly 26 when the controller 30 isprogrammed to activate the motor 162 at lower speeds (speeds less than10 mph) and highway cruising speeds (speeds greater than 55 mph) willnow be described in detail. If vehicle 10 is at a stationary position,or rolling at less than 10 mph, sensor 32B sends a signal to thecontroller 30 as such. Controller 30 also receives a signal from sensor32F indicative of the throttle position. The controller then sendscommands to the motor 162 and the actuator 192 to activate the motor 162and the actuator 192 if vehicle acceleration is requested. Activation ofthe motor 162 causes the rotor 184 to rotate about the third axis ofrotation 175. The activated actuator 192 axially slides the first clutchplates 194 and the second clutch plates 196 into engagement to drivinglyinterconnect the rotor 184 and the second pinion tail 170. Therefore,rotation of the rotor 184 effectuates rotation of the second pinion 160.

The controller 30 also controls the speed at which the rotor 184 and thesecond pinion 160 rotate. During acceleration of the vehicle 10, thesecond pinion 160 exerts a driving force on the ring gear 82 tosupplement the driving force that the first pinion 110 exerts upon thering gear 82. This condition allows decreasing the output required bythe engine 16. To properly operate motor 162, the controller 30identifies the speed of the engine 16, the speed of the ring gear 82 andthe position of the throttle. Use of the motor 162 in this manner todecrease the load on the engine 16 advantageously improves the fueleconomy of the vehicle 10 and prolongs the life of the engine 16.

After the controller 30 determines that the speed of the vehicle 10 isgreater than 10 mph by processing inputs from the sensor 32B, thecontroller 30 sends a signal to the motor 162 to deactivate the motor162. Even after the motor 162 is deactivated, the second pinion 160continues to rotate due to the cooperation between the head 172 and thering gear 82. At this time, motor 162 may be decoupled from secondpinion 160 or may remain coupled to provide braking. For example, thecontroller 30 may be programmed to deactivate actuator 192 to decouplethe second pinion 160 from the rotor 184. Alternatively, if the secondpinion 160 and the rotor 184 remain coupled, then rotation of the secondpinion due to rotation of the ring gear 82 results in rotation of therotor 184, thus causing the motor 162 to act as a generator. Electricitygenerated by the motor 162 can be transferred to the battery 29 during acharging operation. If the second pinion 160 and the rotor 184 aredecoupled then the second pinion 160 freely spins and no electricity isgenerated.

The electric drive assembly 26 can also serve as the sole source ofpropulsion for the vehicle 10. For example, after the vehicle 10 ispropelled to a predetermined cruising speed by the engine 16, such as 55mph, the controller 30 can activate the electric motor 162 to rotate thesecond pinion 160 in the manner set forth above. The controller 30 thencommands the engine 16 to reduce its RPM output to idle and therotational speed of the second pinion 160 is set to provide the entiredriving force necessary to drive the ring gear 82 and propel the vehicle10. At idle, the engine 16 is still operable to maintain the powersteering, power brakes, and other vehicle accessories, but uses far lessfuel and improves the vehicle's fuel economy. To accelerate the vehicle10 after the engine 16 is brought to idle, the controller 30 canincrease the rotational speed of the second pinion 160 and/or increasethe speed of the engine 16 to increase the rotational speed of the firstpinion 110. The motor 162 can also be used to provide the entire drivingforce necessary to drive the ring gear 82 to propel the vehicle 10 froma standstill.

FIG. 4 illustrates an electric drive assembly according to an additionalembodiment at reference numeral 200. The electric drive assembly 200 issimilar to the electric drive assembly 26 and like reference numbers areused to identify like elements of the embodiments with the like elementsof the assembly 200 including the prime (′) designation. The abovedescription of the like elements and their operation equally applies tothe assembly 200.

The assembly 200 is a simplified version of assembly 26 and does notinclude the clutch assembly 164. In the assembly 200, the rotor 184′ ofthe motor 162′ is rotationally fixed to the tail 170′ of the secondpinion 160′. Therefore, rotation of the second pinion 160′ by the ringgear 82 when the motor 162′ is not activated causes the rotor 184′ torotate. Controller 30 is operable to allow motor 162′ to rotate freelyor to function as a generator to generate electricity.

FIG. 5 illustrates an electric drive assembly according to an alternateembodiment at reference numeral 300. The electric drive assembly 300 issimilar to the electric drive assemblies 26 and 200 and like referencenumbers are used to identify like elements of the embodiments with thelike elements of the assembly 300 including the double prime (″)designation. The above description of the like elements equally appliesto the assembly 300.

The assembly 300 is substantially similar to the assembly 200 exceptthat the assembly 300 includes a pinion actuator 302. The actuator 302can be any conventional actuator that is operable to linearly slide thesecond pinion 160″ along the third axis 175″ between a first position A,in which the head 172″ meshingly engages the ring gear 82, and a secondposition B, in which the head 172″ is spaced apart from the ring gear82. In this embodiment, when torque is required from the motor 162″, thecontroller 30 checks the position of the actuator 302 using the sensor32E to make sure that the second pinion 160″ is at position A. If thesecond pinion 160″ is not at position A then the controller activatesthe actuator 302 to move the second pinion 160″ to position A.

When the controller 30 determines that use of the motor 162″ is notnecessary, the controller 30 deactivates the motor 162″. If thecontroller 30 determines that the battery 29, or other device, needs tobe recharged, the controller 30 keeps the second pinion 160″ at positionA so that rotation of the second pinion 160″ by the ring gear 82 willrotate the rotor 184″ and cause the motor 162″ to generate electricitythat can be stored by the battery 29. If the controller 30 determinesthat the motor 162″ does not need to be used to generate electricity,then the controller 30 signals the actuator 302 to move the secondpinion 160″ to position B so that the second pinion 160″ is not indrivingly engaged with the ring gear 82. Use of the assembly 300 isadvantageous because the second pinion 160″ can be moved from engagementwith the ring gear 82 when the motor 162″ is not in use and, therefore,the second pinion 160″ does not cause unnecessary drag on the ring gear82, which can negatively effect fuel economy. The actuator 302 of theassembly 300 can also be provided in the assemblies 26 and 200 describedabove to move the second pinions 160 and 160′ into and out of engagementwith the ring gear 82 as desired.

One of ordinary skill in the art will recognize that the electric driveassemblies 26, 200, and 300 of the present invention can be adapted foruse with most any type of motor vehicle. For example, the assemblies 26,200, and 300 can be adapted to vehicles having front wheel drive, rearwheel drive, two wheel drive, four wheel drive, and all wheel drivesystems. Further, the assemblies 26, 200, and 300 may be mounted toexisting carrier assemblies of previously manufactured vehicles. Also,the assemblies 26, 200, and 300 may be provided with new carrierassemblies of new vehicles.

Therefore, the electric drive assembly of the present invention isoperable to increase the fuel economy of the vehicle 10 and decrease theload on the engine 16 by driving a second pinion 160 that is incooperation with the ring gear 82. The driving force exerted by thesecond pinion 160 replaces or supplements the driving force of the firstpinion 110 and thus the output required of the engine 16 is decreased.The electric motor 162 may be in operation at all times or only atselect times, such as during acceleration when the demands of the engine16 are the greatest or during highway cruising speeds when the electricmotor 162 is capable of solely propelling the vehicle 10 with minimalperformance loss. The electric drive assembly offers such advantageswhile still providing the user with the performance advantages of atraditional combustion engine.

Furthermore, the foregoing discussion discloses and describes merelyexemplary embodiments of the present invention. One skilled in the artwill readily recognize from such discussion, and from the accompanyingdrawings and claims, that various changes, modifications and variationsmay be made therein without department from the spirit and scope of theinvention as defined in the following claims.

1. A motor vehicle, comprising: a powertrain for generating drivetorque; a driveline transmitting drive torque from said powertrain to apair of wheels, said driveline including an axle assembly having a pairof axleshafts coupled to said wheels and a differential unit drivingsaid axleshafts, and a shaft assembly operably coupling said powertrainto said differential unit, said shaft assembly including a first pinionmeshed with a ring gear driving said differential unit; and an electricdrive assembly including a second pinion meshed with said ring gear, anelectric motor and a clutch assembly for selectively interconnectingsaid electric motor to said second pinion, said first and second pinionsaligned to rotate generally about a common axis of rotation.
 2. Themotor vehicle of claim 1 further including an actuator operable toselectively provide an application force to said clutch assembly todrivingly interconnect said electric motor and said second pinion. 3.The motor vehicle of claim 1 wherein said electric drive assembly ismounted to a cover pan opening of a carrier housing associated with saidaxle assembly.
 4. The motor vehicle of claim 1 further comprising acontroller operable to control the operation of said electric motor. 5.The motor vehicle of claim 1 wherein said electric motor selectivelyoperates as a generator to generate electricity.
 6. The motor vehicle ofclaim 1 wherein said second pinion is movable between a first positionin which said second pinion drivingly engages said ring gear and asecond position in which said second pinion is displaced from meshedengagement with said ring gear.
 7. A motor vehicle, comprising: apowertrain for generating drive torque; a driveline transmitting drivetorque from said powertrain to a pair of wheels, said drivelineincluding an axle assembly having a pair of axleshafts coupled to saidwheels and a differential unit driving said axleshafts, and a shaftassembly operably coupling said powertrain to said differential unit,said shaft assembly including a first pinion meshed with a ring geardriving said differential unit; an electric drive assembly including asecond pinion meshed with said ring gear, an electric motor and a clutchassembly for selectively drivingly interconnecting said electric motorto said second pinion, said first and second pinions aligned to rotategenerally about a common axis of rotation; and a controller monitoringoperating parameters of said vehicle and selectively actuating saidelectric motor.
 8. The motor vehicle of claim 7 wherein said electricdrive assembly is mounted to a cover pan opening of a carrier housingassociated with said axle assembly.
 9. A motor vehicle, comprising: apowertrain for generating drive torque; a driveline for transmittingdrive torque from said powertrain to a pair of wheels, said drivelineincluding an axle assembly having a housing defining a central cavityand an opening, a differential unit rotatably supported with saidcentral cavity, a pair of axleshafts coupling said differential unit tosaid wheels, a ring gear fixed to said differential unit and a firstpinion driven by said powertrain and meshed with said ring gear; and anelectric drive assembly including an electric motor, a second pinionextending through said opening in said housing so as to mesh with saidring gear, and a clutch assembly for selectively drivinglyinterconnecting said electric motor to said second pinion.
 10. The motorvehicle of claim 9 wherein said electric drive assembly is mounted tosaid housing.
 11. A motor vehicle, comprising: a powertrain forgenerating drive torque; a driveline for transmitting drive torque fromsaid powertrain to a pair of wheels, said driveline including an axleassembly having a housing, a differential rotatably supported by saidhousing, a ring gear fixed for rotation with said differential, and afirst pinion meshed with said ring gear and driven by said powertrainfor driving said ring gear; and an electric drive assembly mounted tosaid housing, said electric drive assembly including an electric motor,a second pinion meshed with said ring gear, and a clutch assemblyoperable for selectively coupling said electric motor to said secondpinion for driving said ring gear.
 12. A motor vehicle, comprising: apowertrain for generating drive torque; a driveline for transmittingdrive torque from said powertrain to a pair of wheels, said drivelineincluding an axle assembly having a carrier housing, a differentialassembly rotatably supported by said carrier housing, a ring gear fixedfor rotation with said differential assembly, and a first pinion meshedwith said ring gear and driven by said powertrain for driving said ringgear; and an electric drive assembly including an electric motor and asecond pinion selectively driven by said electric motor for driving saidring gear, said electric drive assembly is mounted to a cover a panopening associated with said carrier housing.
 13. A motor vehicle,comprising: a powertrain for generating drive torque; an axle assemblyincluding a housing defining a central cavity and first and secondopenings, a differential rotatably supported in said central cavity,said differential including an input member, a first output memberdriving a first wheel and a second output member driving a second wheel,a ring gear fixed for rotation with said input member of saiddifferential, a first pinion shaft extending through said first openingin said housing and having a first end driven by said powertrain and asecond end defining a first pinion gear that is meshed with said ringgear, and a second pinion shaft extending through said second opening insaid housing and having a first end and a second end defining a secondpinion gear that is meshed with said ring gear; and an electric driveassembly mounted to said housing and including an electric motor and aclutch operable for selectively coupling said electric motor to saidfirst end of said second pinion shaft.
 14. A motor vehicle, comprising:a powertrain for generating drive torque; an axle assembly having ahousing, a differential unit rotatably supported in said housing, a ringgear driving said differential unit, a pair of axleshafts driven by saiddifferential unit, and a first shaft driven by said powertrain andhaving a first pinion driving said ring gear; and an electric driveassembly mounted to a cover pan opening of said housing and including asecond shaft having a second pinion driving said ring gear and anelectric motor driving said second shaft.
 15. The motor vehicle of claim14 wherein said electric drive assembly further includes a clutchassembly for releasably coupling an output of said electric motor tosaid second shaft.